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Old 9th October 2006, 15:54   #1  |  Link
csnet
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How to capture old computer and games RF output

I am starting this thread, hoping that after everyone has chance to post, we define a clear methodology for capturing the output from the RF modulators of old computers and games from the early 1980s that were meant to use a TV set as a monitor.

I'll start with my own issue, the inability to capture the output of an old Sinclair ZX-81 computer. This thread also appplies to Timex-Sinclair, Atari, Commodore, Texas Instruments, Radio Shack, and other computers as well as numerous computer games that used TVs for the display.

Here is the issue:

1. The TV Channel output of an old Sinclair ZX-81 computer produces a picture on a TV screen when connected to the TV's 75 ohm RF inputs.

2. Connecting the TV Channel RF output of the Sinclair ZX-81 computer to the 75 ohm RF input of a capture card initially shows a few frames of the computer output, then the captured video fades to black.

3. Connecting the composite video out of a TV set that is showing the ZX-81 picture to the composite input of the capture card initially shows a few frames of the computer output, then the captured video fades to black.

4. Connecting that TV signal to the 75 ohm RF input of several VCRs and then connecting the composite video out of the VCR to the composite input of the capture card initially shows a few frames of the computer output, then the captured video fades to black.

I have set up a web page http://www.csnetserver.com/computers/supergraphs/ with links, photos, screen shots, and a video of the "initially shows a few frames of the computer output, then the captured video fades to black" experience.

I am hoping an expert out there can tell why the ZX-81 signal can be seen on TVs, yet the 75 ohm composite out of this TV image cannot be seen on a capture card except for a few frames.

Here are some questions:

1. Are capture cards pretty much the same these days in the way they accept and sync up with incoming signals, or are there capture cards that are known to work with the ZX-81 and other computer and games output? The WinFast card used in the tests appears to be using a ConExtant CX238880 chip or equivalent.

2. Are modern capture cards seeing the ZX-81 output as incomplete scan lines and therefore rejecting them? Note the narrow VirtualDub histogram screen shot of the ZX-81 output on the SuperGraphs web page.

3. Would some kind of external mixer or circuit that would blend a full scan black image with the ZX-81 composite signal allow a capture card to sync up with the mixed output and therefore allow capturing the ZX-81 output?

This is my first post here. I hope I have provided enough information for the discussion, If not, please let me know.
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